docs: def _construct_SAX_chunk_dict(self, sax):
          """
          Form a chunk frequency dictionary from a SAX representation.
          :param str sax: a SAX representation.
          :return dict: frequency dictionary for chunks in the SAX representation.
          """
          frequency = defaultdict(int)
          chunk_size = self.chunk_size
          length = len(sax)
          for i in range(length):
              if i + chunk_size <= length:
                  chunk = sax[i: i + chunk_size]
                  frequency[chunk] += 1
          return frequency